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Original question:

BQ20Z95: BQ20z95 FCC Error

BQ20Z95: Incorrect Cycle Count and Capacity

RJ
Intellectual 2550 points

Part Number: BQ20Z95

Hi Team,

Having  an issue I hop you can provide some clarity on. Note that our pack is a 3s9p pack, ~ 25Ah/12.6V. Because the pack Ah is greater than what the bq20z95 can handle (capacity wise), we had to do the / by 2 method.  (Half current, half Pack Ah)

We are seeing a discrepancy in packs that have been out in the field and used for a while between the FCC it reads and what is actually in the pack. We verify that by pulling the BMS board that has been on the pack and installing a new board after the learning cycle and testing the amount of energy that can be removed off the pack.

We would think after the new board has leaned the pack that they should match closely. But they don’t. This is leading us to believe the BQ is telling us that the batteries are in lower states of health than what they truly are. We don’t seem to get this in packs that are brand new suggesting that our gg/senc as well as our battery code seems correct.

Also, When the bq states FCC, at what C rate is it relating the FCC to?

If a pack is just sitting there with no load, would FCC increase? (Overtime – from last reported value in use)

 If the pack has an 8A load and then a 4A load, would FCC increase instantly? Or would it take time? How much time? Would it require cycling?

 One last question, we have a golden file for packs with about ~25Ah. If we took that board and put it on a battery that is older and less capacity, let's say 10Ah. How many cycles would you expect for it to relearn the battery?

Would this be an issue related to cold temp cycling? (https://e2e.ti.com/support/power-management/f/196/t/614760?tisearch=e2e-sitesearch&keymatch=bq20z95%2525252520fcc

  • 0
    TI__Guru** 105590 points

    Hello Ramon,

    I'm not sure I understand how you are testing for the FCC discrepancy, if you take a gauge off a pack from the field that has been learning Ra and Qmax it should be gauging accurately, was it showing jumps in SOC?

    The gauge predicts when the voltage will hit terminate voltage, if there is no jump then I would say it is still gauging accurately.

    If you put a gauge that has learned a new packs parameters, the Ra and Qmax will representative of a new battery and I would expect it would not gauge as accurately until it has learned the parameters of the old pack.

    The FCC will update periodically with no load, it will also update at each Ra grid point, when temperature or current changes by a set amount.

    Sincerely,

    Wyatt Keller

  • 0 in reply to Wyatt Keller
    TI__Intellectual 2550 points

    Hey Wyatt,

    Regarding this statement:

    “..If you put a gauge that has learned a new packs parameters, the Ra and Qmax will representative of a new battery and I would expect it would not gauge as accurately until it has learned the parameters of the old pack.”

    How long could it take for example going from a 20Ah pack to a 10aH pack in cycles?

    Low FCC battery.zipAh pack in cycles?

  • 0 in reply to RJ
    TI__Guru** 105590 points

    Hello Ramon,

    I'm not sure how long it would take as this is not a normal use case for the gauges. If the update status was reset then the full learning cycle completed it should be good after that, but without re-doing the learning cycle I'm not sure how long it would take.

    Sincerely,

    Wyatt Keller